We have prepared isolated hepatocytes from thyroidectomized and sham operated adult rats. There was a decrease in hepatocyte size and protein content per adult rats. There was a decrease in hepatocyte size and protein content per cell in the thyroidectomized animals. When the metablism of acetaminophen is expressed per 10 to the six cells thyroidectomy resulted in an increase in glucuronidation, and decreases in sulfation and glutathione conjugate formation. When the metabolism is expressed per mg of cell protein, glucuronidation was greatly increased in the thyroidectomized animal, sulfation remained little changed and glutathione conjugate formation was decreased. The decrease of glutathione conjugfate formation was also observed in the 9000 x g supernatant of liver to which cofactors were added, indicating a decrease in the cytochrome P-450 specie(s) responsible for its activation. Levels of total cytochrome P-450 were not decreased when expressed per mg of protein. However, increases in phenolsulfotransferase activity were noted in the soluble fraction, so that it appeared that the activity of the transferase was not the limiting factor in acetaminophen sulfation. Administration of thyroid hormones either in vivo or in tissue culture failed to alter either glucuronidation or sulfation, but had complex effects upon glutathione conjugate formation.